iosignalfd

iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling.

Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu.

However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path.

Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components.

To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl().

We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via iosignalfd.

You can download this test harness here:

ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2

The measured results are as follows:

qemu-mmio:       110000 iops, 9.09us rtt
iosignalfd-mmio: 200100 iops, 5.00us rtt
iosignalfd-pio:  367300 iops, 2.72us rtt

I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get:

qemu-pio:      153139 iops, 6.53us rtt
iosignalfd-hc: 412585 iops, 2.37us rtt

these are just for fun, for now, until I can gather more data.

Here is a graph for your convenience:

http://developer.novell.com/wiki/images/7/76/Iofd-chart.png

The conclusion to draw is that we save about 4us by skipping the userspace hop.

--------------------

Signed-off-by Gregory Haskins <[email protected]>

---

 arch/x86/kvm/x86.c       |    1 
 include/linux/kvm.h      |   15 ++
 include/linux/kvm_host.h |   10 +
 virt/kvm/eventfd.c       |  356 ++++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |   11 +
 5 files changed, 389 insertions(+), 4 deletions(-)

arch/x86/kvm/x86.c +1 -0

index c1ed485..c96c0e3 100644
@@ -1097,6 +1097,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IRQ_INJECT_STATUS:
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_IRQFD:
+	case KVM_CAP_IOSIGNALFD:
 	case KVM_CAP_PIT2:
 		r = 1;
 		break;

include/linux/kvm.h +15 -0

index 632a856..53b720d 100644
@@ -300,6 +300,19 @@ struct kvm_guest_debug {
 	struct kvm_guest_debug_arch arch;
 };
 
+#define KVM_IOSIGNALFD_FLAG_TRIGGER   (1 << 0)
+#define KVM_IOSIGNALFD_FLAG_PIO       (1 << 1)
+#define KVM_IOSIGNALFD_FLAG_DEASSIGN  (1 << 2)
+
+struct kvm_iosignalfd {
+	__u64 trigger;
+	__u64 addr;
+	__u32 len;
+	__u32 fd;
+	__u32 flags;
+	__u8  pad[36];
+};
+
 #define KVM_TRC_SHIFT           16
 /*
  * kvm trace categories

@@ -430,6 +443,7 @@ struct kvm_trace_rec {
 #ifdef __KVM_HAVE_PIT
 #define KVM_CAP_PIT2 33
 #endif
+#define KVM_CAP_IOSIGNALFD 34
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -537,6 +551,7 @@ struct kvm_irqfd {
 #define KVM_DEASSIGN_DEV_IRQ       _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
 #define KVM_IRQFD                  _IOW(KVMIO, 0x76, struct kvm_irqfd)
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
+#define KVM_IOSIGNALFD             _IOW(KVMIO, 0x78, struct kvm_iosignalfd)
 
 /*
  * ioctls for vcpu fds

include/linux/kvm_host.h +8 -2

index 216fe07..b705960 100644
@@ -138,6 +138,7 @@ struct kvm {
 	struct kvm_io_bus pio_bus;
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 	struct list_head irqfds;
+	struct list_head iosignalfds;
 #endif
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;

@@ -533,19 +534,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
 
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 
-void kvm_irqfd_init(struct kvm *kvm);
+void kvm_eventfd_init(struct kvm *kvm);
 int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
 void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
 
 #else
 
-static inline void kvm_irqfd_init(struct kvm *kvm) {}
+static inline void kvm_eventfd_init(struct kvm *kvm) {}
 static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
 	return -EINVAL;
 }
 
 static inline void kvm_irqfd_release(struct kvm *kvm) {}
+static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	return -EINVAL;
+}
 
 #endif /* CONFIG_HAVE_KVM_EVENTFD */
 

virt/kvm/eventfd.c +93 -1

index f3f2ea1..77befb3 100644
@@ -21,6 +21,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/kvm.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
 #include <linux/wait.h>

@@ -29,6 +30,8 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 
+#include "iodev.h"
+
 /*
  * --------------------------------------------------------------------
  * irqfd: Allows an fd to be used to inject an interrupt to the guest

@@ -208,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
 }
 
 void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
 {
 	INIT_LIST_HEAD(&kvm->irqfds);
+	INIT_LIST_HEAD(&kvm->iosignalfds);
 }
 
 int

@@ -233,3 +237,353 @@ kvm_irqfd_release(struct kvm *kvm)
 		irqfd_release(irqfd);
 	}
 }
+
+/*
+ * --------------------------------------------------------------------
+ * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+/*
+ * Design note: We create one PIO/MMIO device (iosignalfd_group) which
+ * aggregates  one or more iosignalfd_items.  Each item points to exactly one
+ * eventfd, and can be registered to trigger on any write to the group
+ * (wildcard), or to a write of a specific value.  If more than one item is to
+ * be supported, the addr/len ranges must all be identical in the group.  If a
+ * trigger value is to be supported on a particular item, the group range must
+ * be exactly the width of the trigger.
+ */
+
+struct _iosignalfd_item {
+	struct list_head     list;
+	struct file         *file;
+	unsigned char       *match;
+	struct rcu_head      rcu;
+};
+
+struct _iosignalfd_group {
+	struct list_head     list;
+	u64                  addr;
+	size_t               length;
+	struct list_head     items;
+	struct kvm_io_device dev;
+};
+
+static inline struct _iosignalfd_group *to_group(struct kvm_io_device *dev)
+{
+	return container_of(dev, struct _iosignalfd_group, dev);
+}
+
+static inline struct _iosignalfd_item *to_item(struct rcu_head *rhp)
+{
+	return container_of(rhp, struct _iosignalfd_item, rcu);
+}
+
+static int
+iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
+			  int is_write)
+{
+	struct _iosignalfd_group *p = to_group(this);
+
+	return ((addr >= p->addr && (addr < p->addr + p->length)));
+}
+
+static int
+iosignalfd_is_match(struct _iosignalfd_group *group,
+		    struct _iosignalfd_item *item,
+		    const void *val,
+		    int len)
+{
+	if (!item->match)
+		/* wildcard is a hit */
+		return true;
+
+	if (len != group->length)
+		/* mis-matched length is a miss */
+		return false;
+
+	/* otherwise, we have to actually compare the data */
+	return !memcmp(item->match, val, len) ? true : false;
+}
+
+/*
+ * MMIO/PIO writes trigger an event (if the data matches).
+ *
+ * This is invoked by the io_bus subsystem in response to an address match
+ * against the group.  We must then walk the list of individual items to check
+ * for a match and, if applicable, to send the appropriate signal. If the item
+ * in question does not have a "match" pointer, it is considered a wildcard
+ * and will always generate a signal.  There can be an arbitrary number
+ * of distinct matches or wildcards per group.
+ */
+static void
+iosignalfd_group_write(struct kvm_io_device *this, gpa_t addr, int len,
+		       const void *val)
+{
+	struct _iosignalfd_group *group = to_group(this);
+	struct _iosignalfd_item *item;

virt/kvm/eventfd.c +215 -0

+}
+
+/*
+ * MMIO/PIO reads against the group indiscriminately return all zeros
+ */
+static void
+iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
+		      void *val)
+{
+	memset(val, 0, len);
+}
+
+static void
+_iosignalfd_group_destructor(struct _iosignalfd_group *group)
+{
+	list_del(&group->list);
+	kfree(group);
+}
+
+static void
+iosignalfd_group_destructor(struct kvm_io_device *this)
+{
+	struct _iosignalfd_group *group = to_group(this);
+
+	_iosignalfd_group_destructor(group);
+}
+
+/* assumes kvm->lock held */
+static struct _iosignalfd_group *
+iosignalfd_group_find(struct kvm *kvm, u64 addr)
+{
+	struct _iosignalfd_group *group;
+
+	list_for_each_entry(group, &kvm->iosignalfds, list) {
+		if (group->addr == addr)
+			return group;
+	}
+
+	return NULL;
+}
+
+static const struct kvm_io_device_ops iosignalfd_ops = {
+	.read       = iosignalfd_group_read,
+	.write      = iosignalfd_group_write,
+	.in_range   = iosignalfd_group_in_range,
+	.destructor = iosignalfd_group_destructor,
+};
+
+/*
+ * Atomically find an existing group, or create a new one if it doesn't already
+ * exist.
+ *
+ * assumes kvm->lock is held
+ */
+static struct _iosignalfd_group *
+iosignalfd_group_get(struct kvm *kvm, struct kvm_io_bus *bus,
+		      u64 addr, size_t len)
+{
+	struct _iosignalfd_group *group;
+
+	group = iosignalfd_group_find(kvm, addr);
+	if (!group) {
+		int ret;
+
+		group = kzalloc(sizeof(*group), GFP_KERNEL);
+		if (!group)
+			return ERR_PTR(-ENOMEM);
+
+		INIT_LIST_HEAD(&group->list);
+		INIT_LIST_HEAD(&group->items);
+		group->addr   = addr;
+		group->length = len;
+		kvm_iodevice_init(&group->dev, &iosignalfd_ops);
+
+		ret = kvm_io_bus_register_dev(bus, &group->dev);
+		if (ret < 0) {
+			kfree(group);
+			return ERR_PTR(ret);
+		}
+
+		list_add_tail(&group->list, &kvm->iosignalfds);
+
+	} else if (group->length != len)
+		/*
+		 * Existing groups must have the same addr/len tuple or we
+		 * reject the request
+		 */
+		return ERR_PTR(-EINVAL);
+
+	return group;
+}
+
+static int
+kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group = NULL;
+	struct _iosignalfd_item  *item = NULL;
+	struct file              *file;
+	int                       ret;
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		return ret;
+	}
+
+	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	if (!item) {
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	INIT_LIST_HEAD(&item->list);
+	item->file = file;
+
+	/*
+	 * Registering a "trigger" address is optional.  If this flag
+	 * is not specified, we leave the item->match pointer NULL, which
+	 * indicates a wildcard
+	 */
+	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER) {
+		if (args->len > sizeof(u64)) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		item->match = kzalloc(args->len, GFP_KERNEL);
+		if (!item->match) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		if (copy_from_user(item->match,
+				   (void *)args->trigger,
+				   args->len)) {
+			ret = -EFAULT;
+			goto fail;
+		}
+	}
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_get(kvm, bus, args->addr, args->len);
+	if (IS_ERR(group)) {
+		ret = PTR_ERR(group);
+		mutex_unlock(&kvm->lock);
+		goto fail;
+	}
+
+	/*
+	 * Note: We are committed to succeed at this point since we have
+	 * (potentially) published a new group-device.  Any failure handling
+	 * added in the future after this point will need to be handled
+	 * carefully.
+	 */
+
+	list_add_tail_rcu(&item->list, &group->items);
+
+	mutex_unlock(&kvm->lock);
+
+	return 0;
+
+fail:
+	if (item) {
+		/*
+		 * it would have never made it to the group->items list
+		 * in the failure path, so we dont need to worry about removing
+		 * it
+		 */
+		kfree(item->match);
+		kfree(item);
+	}
+
+	if (file)
+		fput(file);
+
+	return ret;
+}
+
+static void
+iosignalfd_item_free(struct rcu_head *rhp)
+{
+	struct _iosignalfd_item *item = to_item(rhp);
+
+	fput(item->file);
+	kfree(item->match);
+	kfree(item);
+}
+
+static int
+kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group;
+	struct _iosignalfd_item  *item, *tmp;
+	struct file              *file;
+	int                       ret = 0;
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_find(kvm, args->addr);
+	if (!group) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out;
+	}
+

virt/kvm/eventfd.c +27 -0

+	}
+
+	if (list_empty(&group->items)) {
+		/*
+		 * We should unpublish our group device if we just removed
+		 * the last of its contained items
+		 */
+		kvm_io_bus_unregister_dev(bus, &group->dev);
+		_iosignalfd_group_destructor(group);
+	}
+
+	fput(file);
+
+out:
+	mutex_unlock(&kvm->lock);
+
+	return ret;
+}
+
+int
+kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
+		return kvm_deassign_iosignalfd(kvm, args);
+
+	return kvm_assign_iosignalfd(kvm, args);
+}

virt/kvm/kvm_main.c +10 -2

index 179c650..91d0fe2 100644
@@ -977,7 +977,7 @@ static struct kvm *kvm_create_vm(void)
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
-	kvm_irqfd_init(kvm);
+	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
 	init_rwsem(&kvm->slots_lock);

@@ -2215,6 +2215,15 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
 		break;
 	}
+	case KVM_IOSIGNALFD: {
+		struct kvm_iosignalfd entry;
+
+		r = -EFAULT;
+		if (copy_from_user(&entry, argp, sizeof entry))
+			goto out;
+		r = kvm_iosignalfd(kvm, &entry);
+		break;
+	}
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
 	}
 
--

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iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling.

Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu.

However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path.

Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components.

To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl().

We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via iosignalfd.

You can download this test harness here:

ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2

The measured results are as follows:

qemu-mmio:       110000 iops, 9.09us rtt
iosignalfd-mmio: 200100 iops, 5.00us rtt
iosignalfd-pio:  367300 iops, 2.72us rtt

I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get:

qemu-pio:      153139 iops, 6.53us rtt
iosignalfd-hc: 412585 iops, 2.37us rtt

these are just for fun, for now, until I can gather more data.

Here is a graph for your convenience:

http://developer.novell.com/wiki/images/7/76/Iofd-chart.png

The conclusion to draw is that we save about 4us by skipping the userspace hop.

--------------------

Signed-off-by Gregory Haskins <[email protected]>

---

 arch/x86/kvm/x86.c       |    1 
 include/linux/kvm.h      |   15 ++
 include/linux/kvm_host.h |   10 +
 virt/kvm/eventfd.c       |  356 ++++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |   11 +
 5 files changed, 389 insertions(+), 4 deletions(-)

arch/x86/kvm/x86.c +1 -0

index c1ed485..c96c0e3 100644
@@ -1097,6 +1097,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IRQ_INJECT_STATUS:
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_IRQFD:
+	case KVM_CAP_IOSIGNALFD:
 	case KVM_CAP_PIT2:
 		r = 1;
 		break;

include/linux/kvm.h +15 -0

index 632a856..53b720d 100644
@@ -300,6 +300,19 @@ struct kvm_guest_debug {
 	struct kvm_guest_debug_arch arch;
 };
 
+#define KVM_IOSIGNALFD_FLAG_TRIGGER   (1 << 0)
+#define KVM_IOSIGNALFD_FLAG_PIO       (1 << 1)
+#define KVM_IOSIGNALFD_FLAG_DEASSIGN  (1 << 2)
+
+struct kvm_iosignalfd {
+	__u64 trigger;
+	__u64 addr;
+	__u32 len;
+	__u32 fd;
+	__u32 flags;
+	__u8  pad[36];
+};
+
 #define KVM_TRC_SHIFT           16
 /*
  * kvm trace categories

@@ -430,6 +443,7 @@ struct kvm_trace_rec {
 #ifdef __KVM_HAVE_PIT
 #define KVM_CAP_PIT2 33
 #endif
+#define KVM_CAP_IOSIGNALFD 34
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -537,6 +551,7 @@ struct kvm_irqfd {
 #define KVM_DEASSIGN_DEV_IRQ       _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
 #define KVM_IRQFD                  _IOW(KVMIO, 0x76, struct kvm_irqfd)
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
+#define KVM_IOSIGNALFD             _IOW(KVMIO, 0x78, struct kvm_iosignalfd)
 
 /*
  * ioctls for vcpu fds

include/linux/kvm_host.h +8 -2

index 216fe07..b705960 100644
@@ -138,6 +138,7 @@ struct kvm {
 	struct kvm_io_bus pio_bus;
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 	struct list_head irqfds;
+	struct list_head iosignalfds;
 #endif
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;

@@ -533,19 +534,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
 
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 
-void kvm_irqfd_init(struct kvm *kvm);
+void kvm_eventfd_init(struct kvm *kvm);
 int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
 void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
 
 #else
 
-static inline void kvm_irqfd_init(struct kvm *kvm) {}
+static inline void kvm_eventfd_init(struct kvm *kvm) {}
 static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
 	return -EINVAL;
 }
 
 static inline void kvm_irqfd_release(struct kvm *kvm) {}
+static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	return -EINVAL;
+}
 
 #endif /* CONFIG_HAVE_KVM_EVENTFD */
 

virt/kvm/eventfd.c +327 -1

index f3f2ea1..77befb3 100644
@@ -21,6 +21,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/kvm.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
 #include <linux/wait.h>

@@ -29,6 +30,8 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 
+#include "iodev.h"
+
 /*
  * --------------------------------------------------------------------
  * irqfd: Allows an fd to be used to inject an interrupt to the guest

@@ -208,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
 }
 
 void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
 {
 	INIT_LIST_HEAD(&kvm->irqfds);
+	INIT_LIST_HEAD(&kvm->iosignalfds);
 }
 
 int

@@ -233,3 +237,353 @@ kvm_irqfd_release(struct kvm *kvm)
 		irqfd_release(irqfd);
 	}
 }
+
+/*
+ * --------------------------------------------------------------------
+ * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+/*
+ * Design note: We create one PIO/MMIO device (iosignalfd_group) which
+ * aggregates  one or more iosignalfd_items.  Each item points to exactly one
+ * eventfd, and can be registered to trigger on any write to the group
+ * (wildcard), or to a write of a specific value.  If more than one item is to
+ * be supported, the addr/len ranges must all be identical in the group.  If a
+ * trigger value is to be supported on a particular item, the group range must
+ * be exactly the width of the trigger.
+ */
+
+struct _iosignalfd_item {
+	struct list_head     list;
+	struct file         *file;
+	unsigned char       *match;
+	struct rcu_head      rcu;
+};
+
+struct _iosignalfd_group {
+	struct list_head     list;
+	u64                  addr;
+	size_t               length;
+	struct list_head     items;
+	struct kvm_io_device dev;
+};
+
+static inline struct _iosignalfd_group *to_group(struct kvm_io_device *dev)
+{
+	return container_of(dev, struct _iosignalfd_group, dev);
+}
+
+static inline struct _iosignalfd_item *to_item(struct rcu_head *rhp)
+{
+	return container_of(rhp, struct _iosignalfd_item, rcu);
+}
+
+static int
+iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
+			  int is_write)
+{
+	struct _iosignalfd_group *p = to_group(this);
+
+	return ((addr >= p->addr && (addr < p->addr + p->length)));
+}
+
+static int
+iosignalfd_is_match(struct _iosignalfd_group *group,
+		    struct _iosignalfd_item *item,
+		    const void *val,
+		    int len)
+{
+	if (!item->match)
+		/* wildcard is a hit */
+		return true;
+
+	if (len != group->length)
+		/* mis-matched length is a miss */
+		return false;
+
+	/* otherwise, we have to actually compare the data */
+	return !memcmp(item->match, val, len) ? true : false;
+}
+
+/*
+ * MMIO/PIO writes trigger an event (if the data matches).
+ *
+ * This is invoked by the io_bus subsystem in response to an address match
+ * against the group.  We must then walk the list of individual items to check
+ * for a match and, if applicable, to send the appropriate signal. If the item
+ * in question does not have a "match" pointer, it is considered a wildcard
+ * and will always generate a signal.  There can be an arbitrary number
+ * of distinct matches or wildcards per group.
+ */
+static void
+iosignalfd_group_write(struct kvm_io_device *this, gpa_t addr, int len,
+		       const void *val)
+{
+	struct _iosignalfd_group *group = to_group(this);
+	struct _iosignalfd_item *item;
+
+	/* FIXME: We should probably use SRCU */
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(item, &group->items, list) {
+		if (iosignalfd_is_match(group, item, val, len))
+			eventfd_signal(item->file, 1);
+	}
+
+	rcu_read_unlock();
+}
+
+/*
+ * MMIO/PIO reads against the group indiscriminately return all zeros
+ */
+static void
+iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
+		      void *val)
+{
+	memset(val, 0, len);
+}
+
+static void
+_iosignalfd_group_destructor(struct _iosignalfd_group *group)
+{
+	list_del(&group->list);
+	kfree(group);
+}
+
+static void
+iosignalfd_group_destructor(struct kvm_io_device *this)
+{
+	struct _iosignalfd_group *group = to_group(this);
+
+	_iosignalfd_group_destructor(group);
+}
+
+/* assumes kvm->lock held */
+static struct _iosignalfd_group *
+iosignalfd_group_find(struct kvm *kvm, u64 addr)
+{
+	struct _iosignalfd_group *group;
+
+	list_for_each_entry(group, &kvm->iosignalfds, list) {
+		if (group->addr == addr)
+			return group;
+	}
+
+	return NULL;
+}
+
+static const struct kvm_io_device_ops iosignalfd_ops = {
+	.read       = iosignalfd_group_read,
+	.write      = iosignalfd_group_write,
+	.in_range   = iosignalfd_group_in_range,
+	.destructor = iosignalfd_group_destructor,
+};
+
+/*
+ * Atomically find an existing group, or create a new one if it doesn't already
+ * exist.
+ *
+ * assumes kvm->lock is held
+ */
+static struct _iosignalfd_group *
+iosignalfd_group_get(struct kvm *kvm, struct kvm_io_bus *bus,
+		      u64 addr, size_t len)
+{
+	struct _iosignalfd_group *group;
+
+	group = iosignalfd_group_find(kvm, addr);
+	if (!group) {
+		int ret;
+
+		group = kzalloc(sizeof(*group), GFP_KERNEL);
+		if (!group)
+			return ERR_PTR(-ENOMEM);
+
+		INIT_LIST_HEAD(&group->list);
+		INIT_LIST_HEAD(&group->items);
+		group->addr   = addr;
+		group->length = len;
+		kvm_iodevice_init(&group->dev, &iosignalfd_ops);
+
+		ret = kvm_io_bus_register_dev(bus, &group->dev);
+		if (ret < 0) {
+			kfree(group);
+			return ERR_PTR(ret);
+		}
+
+		list_add_tail(&group->list, &kvm->iosignalfds);
+
+	} else if (group->length != len)
+		/*
+		 * Existing groups must have the same addr/len tuple or we
+		 * reject the request
+		 */
+		return ERR_PTR(-EINVAL);
+
+	return group;
+}
+
+static int
+kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group = NULL;
+	struct _iosignalfd_item  *item = NULL;
+	struct file              *file;
+	int                       ret;
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		return ret;
+	}
+
+	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	if (!item) {
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	INIT_LIST_HEAD(&item->list);
+	item->file = file;
+
+	/*
+	 * Registering a "trigger" address is optional.  If this flag
+	 * is not specified, we leave the item->match pointer NULL, which
+	 * indicates a wildcard
+	 */
+	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER) {
+		if (args->len > sizeof(u64)) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		item->match = kzalloc(args->len, GFP_KERNEL);
+		if (!item->match) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		if (copy_from_user(item->match,
+				   (void *)args->trigger,
+				   args->len)) {
+			ret = -EFAULT;
+			goto fail;
+		}
+	}
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_get(kvm, bus, args->addr, args->len);
+	if (IS_ERR(group)) {
+		ret = PTR_ERR(group);
+		mutex_unlock(&kvm->lock);
+		goto fail;
+	}
+
+	/*
+	 * Note: We are committed to succeed at this point since we have
+	 * (potentially) published a new group-device.  Any failure handling
+	 * added in the future after this point will need to be handled
+	 * carefully.
+	 */
+
+	list_add_tail_rcu(&item->list, &group->items);
+
+	mutex_unlock(&kvm->lock);
+
+	return 0;
+
+fail:
+	if (item) {
+		/*
+		 * it would have never made it to the group->items list
+		 * in the failure path, so we dont need to worry about removing
+		 * it
+		 */
+		kfree(item->match);
+		kfree(item);
+	}
+
+	if (file)
+		fput(file);
+
+	return ret;
+}
+
+static void
+iosignalfd_item_free(struct rcu_head *rhp)
+{
+	struct _iosignalfd_item *item = to_item(rhp);
+
+	fput(item->file);
+	kfree(item->match);
+	kfree(item);
+}
+
+static int
+kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group;
+	struct _iosignalfd_item  *item, *tmp;
+	struct file              *file;
+	int                       ret = 0;
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_find(kvm, args->addr);
+	if (!group) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out;
+	}
+
+	list_for_each_entry_safe(item, tmp, &group->items, list) {
+		/*
+		 * any items registered at this group-address with the matching
+		 * eventfd will be removed
+		 */
+		if (item->file != file)
+			continue;
+
+		list_del_rcu(&item->list);

virt/kvm/eventfd.c +18 -0

+	}
+
+	fput(file);
+
+out:
+	mutex_unlock(&kvm->lock);
+
+	return ret;
+}
+
+int
+kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
+		return kvm_deassign_iosignalfd(kvm, args);
+
+	return kvm_assign_iosignalfd(kvm, args);
+}

virt/kvm/kvm_main.c +10 -2

index 179c650..91d0fe2 100644
@@ -977,7 +977,7 @@ static struct kvm *kvm_create_vm(void)
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
-	kvm_irqfd_init(kvm);
+	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
 	init_rwsem(&kvm->slots_lock);

@@ -2215,6 +2215,15 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
 		break;
 	}
+	case KVM_IOSIGNALFD: {
+		struct kvm_iosignalfd entry;
+
+		r = -EFAULT;
+		if (copy_from_user(&entry, argp, sizeof entry))
+			goto out;
+		r = kvm_iosignalfd(kvm, &entry);
+		break;
+	}
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
 	}
 
--

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iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling.

Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu.

However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path.

Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components.

To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl().

We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via iosignalfd.

You can download this test harness here:

ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2

The measured results are as follows:

qemu-mmio:       110000 iops, 9.09us rtt
iosignalfd-mmio: 200100 iops, 5.00us rtt
iosignalfd-pio:  367300 iops, 2.72us rtt

I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get:

qemu-pio:      153139 iops, 6.53us rtt
iosignalfd-hc: 412585 iops, 2.37us rtt

these are just for fun, for now, until I can gather more data.

Here is a graph for your convenience:

http://developer.novell.com/wiki/images/7/76/Iofd-chart.png

The conclusion to draw is that we save about 4us by skipping the userspace hop.

--------------------

Signed-off-by Gregory Haskins <[email protected]>

---

 arch/x86/kvm/x86.c       |    1 
 include/linux/kvm.h      |   15 ++
 include/linux/kvm_host.h |   10 +
 virt/kvm/eventfd.c       |  356 ++++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |   11 +
 5 files changed, 389 insertions(+), 4 deletions(-)

arch/x86/kvm/x86.c +1 -0

index c1ed485..c96c0e3 100644
@@ -1097,6 +1097,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IRQ_INJECT_STATUS:
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_IRQFD:
+	case KVM_CAP_IOSIGNALFD:
 	case KVM_CAP_PIT2:
 		r = 1;
 		break;

include/linux/kvm.h +15 -0

index 632a856..53b720d 100644
@@ -300,6 +300,19 @@ struct kvm_guest_debug {
 	struct kvm_guest_debug_arch arch;
 };
 
+#define KVM_IOSIGNALFD_FLAG_TRIGGER   (1 << 0)
+#define KVM_IOSIGNALFD_FLAG_PIO       (1 << 1)
+#define KVM_IOSIGNALFD_FLAG_DEASSIGN  (1 << 2)
+
+struct kvm_iosignalfd {
+	__u64 trigger;
+	__u64 addr;
+	__u32 len;
+	__u32 fd;
+	__u32 flags;
+	__u8  pad[36];
+};
+
 #define KVM_TRC_SHIFT           16
 /*
  * kvm trace categories

@@ -430,6 +443,7 @@ struct kvm_trace_rec {
 #ifdef __KVM_HAVE_PIT
 #define KVM_CAP_PIT2 33
 #endif
+#define KVM_CAP_IOSIGNALFD 34
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -537,6 +551,7 @@ struct kvm_irqfd {
 #define KVM_DEASSIGN_DEV_IRQ       _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
 #define KVM_IRQFD                  _IOW(KVMIO, 0x76, struct kvm_irqfd)
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
+#define KVM_IOSIGNALFD             _IOW(KVMIO, 0x78, struct kvm_iosignalfd)
 
 /*
  * ioctls for vcpu fds

include/linux/kvm_host.h +8 -2

index 216fe07..b705960 100644
@@ -138,6 +138,7 @@ struct kvm {
 	struct kvm_io_bus pio_bus;
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 	struct list_head irqfds;
+	struct list_head iosignalfds;
 #endif
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;

@@ -533,19 +534,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
 
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 
-void kvm_irqfd_init(struct kvm *kvm);
+void kvm_eventfd_init(struct kvm *kvm);
 int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
 void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
 
 #else
 
-static inline void kvm_irqfd_init(struct kvm *kvm) {}
+static inline void kvm_eventfd_init(struct kvm *kvm) {}
 static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
 	return -EINVAL;
 }
 
 static inline void kvm_irqfd_release(struct kvm *kvm) {}
+static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	return -EINVAL;
+}
 
 #endif /* CONFIG_HAVE_KVM_EVENTFD */
 

virt/kvm/eventfd.c +39 -1

index f3f2ea1..77befb3 100644
@@ -21,6 +21,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/kvm.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
 #include <linux/wait.h>

@@ -29,6 +30,8 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 
+#include "iodev.h"
+
 /*
  * --------------------------------------------------------------------
  * irqfd: Allows an fd to be used to inject an interrupt to the guest

@@ -208,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
 }
 
 void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
 {
 	INIT_LIST_HEAD(&kvm->irqfds);
+	INIT_LIST_HEAD(&kvm->iosignalfds);
 }
 
 int

@@ -233,3 +237,353 @@ kvm_irqfd_release(struct kvm *kvm)
 		irqfd_release(irqfd);
 	}
 }
+
+/*
+ * --------------------------------------------------------------------
+ * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+/*
+ * Design note: We create one PIO/MMIO device (iosignalfd_group) which
+ * aggregates  one or more iosignalfd_items.  Each item points to exactly one
+ * eventfd, and can be registered to trigger on any write to the group
+ * (wildcard), or to a write of a specific value.  If more than one item is to
+ * be supported, the addr/len ranges must all be identical in the group.  If a
+ * trigger value is to be supported on a particular item, the group range must
+ * be exactly the width of the trigger.
+ */
+
+struct _iosignalfd_item {
+	struct list_head     list;
+	struct file         *file;
+	unsigned char       *match;
+	struct rcu_head      rcu;
+};
+
+struct _iosignalfd_group {
+	struct list_head     list;
+	u64                  addr;
+	size_t               length;
+	struct list_head     items;
+	struct kvm_io_device dev;
+};

virt/kvm/eventfd.c +35 -0

+static int
+iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
+			  int is_write)
+{
+	struct _iosignalfd_group *p = to_group(this);
+
+	return ((addr >= p->addr && (addr < p->addr + p->length)));
+}
+
+static int
+iosignalfd_is_match(struct _iosignalfd_group *group,
+		    struct _iosignalfd_item *item,
+		    const void *val,
+		    int len)
+{
+	if (!item->match)
+		/* wildcard is a hit */
+		return true;
+
+	if (len != group->length)
+		/* mis-matched length is a miss */
+		return false;
+
+	/* otherwise, we have to actually compare the data */
+	return !memcmp(item->match, val, len) ? true : false;
+}
+
+/*
+ * MMIO/PIO writes trigger an event (if the data matches).
+ *
+ * This is invoked by the io_bus subsystem in response to an address match
+ * against the group.  We must then walk the list of individual items to check
+ * for a match and, if applicable, to send the appropriate signal. If the item
+ * in question does not have a "match" pointer, it is considered a wildcard
+ * and will always generate a signal.  There can be an arbitrary number

virt/kvm/eventfd.c +5 -0

+	rcu_read_lock();
+
+	list_for_each_entry_rcu(item, &group->items, list) {
+		if (iosignalfd_is_match(group, item, val, len))
+			eventfd_signal(item->file, 1);

From what I can see, this code could invoke eventfd_signal() on an

virt/kvm/eventfd.c +152 -0

+	}
+
+	rcu_read_unlock();
+}
+
+/*
+ * MMIO/PIO reads against the group indiscriminately return all zeros
+ */
+static void
+iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
+		      void *val)
+{
+	memset(val, 0, len);
+}
+
+static void
+_iosignalfd_group_destructor(struct _iosignalfd_group *group)
+{
+	list_del(&group->list);
+	kfree(group);
+}
+
+static void
+iosignalfd_group_destructor(struct kvm_io_device *this)
+{
+	struct _iosignalfd_group *group = to_group(this);
+
+	_iosignalfd_group_destructor(group);
+}
+
+/* assumes kvm->lock held */
+static struct _iosignalfd_group *
+iosignalfd_group_find(struct kvm *kvm, u64 addr)
+{
+	struct _iosignalfd_group *group;
+
+	list_for_each_entry(group, &kvm->iosignalfds, list) {
+		if (group->addr == addr)
+			return group;
+	}
+
+	return NULL;
+}
+
+static const struct kvm_io_device_ops iosignalfd_ops = {
+	.read       = iosignalfd_group_read,
+	.write      = iosignalfd_group_write,
+	.in_range   = iosignalfd_group_in_range,
+	.destructor = iosignalfd_group_destructor,
+};
+
+/*
+ * Atomically find an existing group, or create a new one if it doesn't already
+ * exist.
+ *
+ * assumes kvm->lock is held
+ */
+static struct _iosignalfd_group *
+iosignalfd_group_get(struct kvm *kvm, struct kvm_io_bus *bus,
+		      u64 addr, size_t len)
+{
+	struct _iosignalfd_group *group;
+
+	group = iosignalfd_group_find(kvm, addr);
+	if (!group) {
+		int ret;
+
+		group = kzalloc(sizeof(*group), GFP_KERNEL);
+		if (!group)
+			return ERR_PTR(-ENOMEM);
+
+		INIT_LIST_HEAD(&group->list);
+		INIT_LIST_HEAD(&group->items);
+		group->addr   = addr;
+		group->length = len;
+		kvm_iodevice_init(&group->dev, &iosignalfd_ops);
+
+		ret = kvm_io_bus_register_dev(bus, &group->dev);
+		if (ret < 0) {
+			kfree(group);
+			return ERR_PTR(ret);
+		}
+
+		list_add_tail(&group->list, &kvm->iosignalfds);
+
+	} else if (group->length != len)
+		/*
+		 * Existing groups must have the same addr/len tuple or we
+		 * reject the request
+		 */
+		return ERR_PTR(-EINVAL);
+
+	return group;
+}
+
+static int
+kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group = NULL;
+	struct _iosignalfd_item  *item = NULL;
+	struct file              *file;
+	int                       ret;
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		return ret;
+	}
+
+	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	if (!item) {
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	INIT_LIST_HEAD(&item->list);
+	item->file = file;
+
+	/*
+	 * Registering a "trigger" address is optional.  If this flag
+	 * is not specified, we leave the item->match pointer NULL, which
+	 * indicates a wildcard
+	 */
+	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER) {
+		if (args->len > sizeof(u64)) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		item->match = kzalloc(args->len, GFP_KERNEL);
+		if (!item->match) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		if (copy_from_user(item->match,
+				   (void *)args->trigger,
+				   args->len)) {
+			ret = -EFAULT;
+			goto fail;
+		}
+	}
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_get(kvm, bus, args->addr, args->len);
+	if (IS_ERR(group)) {
+		ret = PTR_ERR(group);
+		mutex_unlock(&kvm->lock);
+		goto fail;

virt/kvm/eventfd.c +54 -0

+	mutex_unlock(&kvm->lock);
+
+	return 0;
+
+fail:
+	if (item) {
+		/*
+		 * it would have never made it to the group->items list
+		 * in the failure path, so we dont need to worry about removing
+		 * it
+		 */
+		kfree(item->match);
+		kfree(item);
+	}
+
+	if (file)
+		fput(file);
+
+	return ret;
+}
+
+static void
+iosignalfd_item_free(struct rcu_head *rhp)
+{
+	struct _iosignalfd_item *item = to_item(rhp);
+
+	fput(item->file);
+	kfree(item->match);
+	kfree(item);
+}
+
+static int
+kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group;
+	struct _iosignalfd_item  *item, *tmp;
+	struct file              *file;
+	int                       ret = 0;
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_find(kvm, args->addr);
+	if (!group) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out;
+	}

virt/kvm/eventfd.c +27 -0

+	}
+
+	if (list_empty(&group->items)) {
+		/*
+		 * We should unpublish our group device if we just removed
+		 * the last of its contained items
+		 */
+		kvm_io_bus_unregister_dev(bus, &group->dev);
+		_iosignalfd_group_destructor(group);
+	}
+
+	fput(file);
+
+out:
+	mutex_unlock(&kvm->lock);
+
+	return ret;
+}
+
+int
+kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
+		return kvm_deassign_iosignalfd(kvm, args);
+
+	return kvm_assign_iosignalfd(kvm, args);
+}

virt/kvm/kvm_main.c +10 -1

index 179c650..91d0fe2 100644
@@ -977,7 +977,7 @@ static struct kvm *kvm_create_vm(void)
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
-	kvm_irqfd_init(kvm);
+	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
 	init_rwsem(&kvm->slots_lock);

@@ -2215,6 +2215,15 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
 		break;
 	}
+	case KVM_IOSIGNALFD: {
+		struct kvm_iosignalfd entry;
+
+		r = -EFAULT;
+		if (copy_from_user(&entry, argp, sizeof entry))
+			goto out;
+		r = kvm_iosignalfd(kvm, &entry);
+		break;
+	}
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
 	}

Quoted 57 lines · Gregory Haskins

iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling.

Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu.

However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path.

Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components.

To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl().

We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via iosignalfd.

You can download this test harness here:

ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2

The measured results are as follows:

qemu-mmio:       110000 iops, 9.09us rtt
iosignalfd-mmio: 200100 iops, 5.00us rtt
iosignalfd-pio:  367300 iops, 2.72us rtt

I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get:

qemu-pio:      153139 iops, 6.53us rtt
iosignalfd-hc: 412585 iops, 2.37us rtt

these are just for fun, for now, until I can gather more data.

Quoted 9 lines

Here is a graph for your convenience:

http://developer.novell.com/wiki/images/7/76/Iofd-chart.png

The conclusion to draw is that we save about 4us by skipping the userspace hop.

--------------------

One set of decision criteria for RCU vs. SRCU below, and one question about the RCU reader running concurrently with a deletion. Looks pretty good, in general!

Thanx, Paul

Quoted 179 lines

Signed-off-by Gregory Haskins <[email protected]>

---

 arch/x86/kvm/x86.c       |    1 
 include/linux/kvm.h      |   15 ++
 include/linux/kvm_host.h |   10 +
 virt/kvm/eventfd.c       |  356 ++++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |   11 +
 5 files changed, 389 insertions(+), 4 deletions(-)

arch/x86/kvm/x86.c +1 -0

index c1ed485..c96c0e3 100644
@@ -1097,6 +1097,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IRQ_INJECT_STATUS:
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_IRQFD:
+	case KVM_CAP_IOSIGNALFD:
 	case KVM_CAP_PIT2:
 		r = 1;
 		break;

include/linux/kvm.h +15 -0

index 632a856..53b720d 100644
@@ -300,6 +300,19 @@ struct kvm_guest_debug {
 	struct kvm_guest_debug_arch arch;
 };
 
+#define KVM_IOSIGNALFD_FLAG_TRIGGER   (1 << 0)
+#define KVM_IOSIGNALFD_FLAG_PIO       (1 << 1)
+#define KVM_IOSIGNALFD_FLAG_DEASSIGN  (1 << 2)
+
+struct kvm_iosignalfd {
+	__u64 trigger;
+	__u64 addr;
+	__u32 len;
+	__u32 fd;
+	__u32 flags;
+	__u8  pad[36];
+};
+
 #define KVM_TRC_SHIFT           16
 /*
  * kvm trace categories

@@ -430,6 +443,7 @@ struct kvm_trace_rec {
 #ifdef __KVM_HAVE_PIT
 #define KVM_CAP_PIT2 33
 #endif
+#define KVM_CAP_IOSIGNALFD 34
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -537,6 +551,7 @@ struct kvm_irqfd {
 #define KVM_DEASSIGN_DEV_IRQ       _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
 #define KVM_IRQFD                  _IOW(KVMIO, 0x76, struct kvm_irqfd)
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
+#define KVM_IOSIGNALFD             _IOW(KVMIO, 0x78, struct kvm_iosignalfd)
 
 /*
  * ioctls for vcpu fds

include/linux/kvm_host.h +8 -2

index 216fe07..b705960 100644
@@ -138,6 +138,7 @@ struct kvm {
 	struct kvm_io_bus pio_bus;
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 	struct list_head irqfds;
+	struct list_head iosignalfds;
 #endif
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;

@@ -533,19 +534,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
 
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 
-void kvm_irqfd_init(struct kvm *kvm);
+void kvm_eventfd_init(struct kvm *kvm);
 int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
 void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
 
 #else
 
-static inline void kvm_irqfd_init(struct kvm *kvm) {}
+static inline void kvm_eventfd_init(struct kvm *kvm) {}
 static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
 	return -EINVAL;
 }
 
 static inline void kvm_irqfd_release(struct kvm *kvm) {}
+static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	return -EINVAL;
+}
 
 #endif /* CONFIG_HAVE_KVM_EVENTFD */
 

virt/kvm/eventfd.c +46 -1

index f3f2ea1..77befb3 100644
@@ -21,6 +21,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/kvm.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
 #include <linux/wait.h>

@@ -29,6 +30,8 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 
+#include "iodev.h"
+
 /*
  * --------------------------------------------------------------------
  * irqfd: Allows an fd to be used to inject an interrupt to the guest

@@ -208,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
 }
 
 void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
 {
 	INIT_LIST_HEAD(&kvm->irqfds);
+	INIT_LIST_HEAD(&kvm->iosignalfds);
 }
 
 int

@@ -233,3 +237,353 @@ kvm_irqfd_release(struct kvm *kvm)
 		irqfd_release(irqfd);
 	}
 }
+
+/*
+ * --------------------------------------------------------------------
+ * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+/*
+ * Design note: We create one PIO/MMIO device (iosignalfd_group) which
+ * aggregates  one or more iosignalfd_items.  Each item points to exactly one
+ * eventfd, and can be registered to trigger on any write to the group
+ * (wildcard), or to a write of a specific value.  If more than one item is to
+ * be supported, the addr/len ranges must all be identical in the group.  If a
+ * trigger value is to be supported on a particular item, the group range must
+ * be exactly the width of the trigger.
+ */
+
+struct _iosignalfd_item {
+	struct list_head     list;
+	struct file         *file;
+	unsigned char       *match;
+	struct rcu_head      rcu;
+};
+
+struct _iosignalfd_group {
+	struct list_head     list;
+	u64                  addr;
+	size_t               length;
+	struct list_head     items;
+	struct kvm_io_device dev;
+};
+
+static inline struct _iosignalfd_group *to_group(struct kvm_io_device *dev)
+{
+	return container_of(dev, struct _iosignalfd_group, dev);
+}
+
+static inline struct _iosignalfd_item *to_item(struct rcu_head *rhp)

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 283

Quoted 4 lines

virt/kvm/eventfd.c +3 -0

+{
+	return container_of(rhp, struct _iosignalfd_item, rcu);
+}

Given that you only use to_item() in one place, not clear to me that it is a win compared to open-coding it. In contrast, to_group() is used in several places, so it makes sense to define it.

Quoted 37 lines

virt/kvm/eventfd.c +35 -0

+static int
+iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
+			  int is_write)
+{
+	struct _iosignalfd_group *p = to_group(this);
+
+	return ((addr >= p->addr && (addr < p->addr + p->length)));
+}
+
+static int
+iosignalfd_is_match(struct _iosignalfd_group *group,
+		    struct _iosignalfd_item *item,
+		    const void *val,
+		    int len)
+{
+	if (!item->match)
+		/* wildcard is a hit */
+		return true;
+
+	if (len != group->length)
+		/* mis-matched length is a miss */
+		return false;
+
+	/* otherwise, we have to actually compare the data */
+	return !memcmp(item->match, val, len) ? true : false;
+}
+
+/*
+ * MMIO/PIO writes trigger an event (if the data matches).
+ *
+ * This is invoked by the io_bus subsystem in response to an address match
+ * against the group.  We must then walk the list of individual items to check
+ * for a match and, if applicable, to send the appropriate signal. If the item
+ * in question does not have a "match" pointer, it is considered a wildcard
+ * and will always generate a signal.  There can be an arbitrary number

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 329

Quoted 10 lines

virt/kvm/eventfd.c +10 -0

+ * of distinct matches or wildcards per group.
+ */
+static void
+iosignalfd_group_write(struct kvm_io_device *this, gpa_t addr, int len,
+		       const void *val)
+{
+	struct _iosignalfd_group *group = to_group(this);
+	struct _iosignalfd_item *item;
+
+	/* FIXME: We should probably use SRCU */

Some test questions to help with this decision:

1. Ignoring lock-contention issues, would you be willing to hold a
spinlock_t across this critical section? If the answer is "yes", then you should also be willing to use rcu_read_lock(). If the answer is "no", proceed to the next question.

2. Is this critical section's execution time almost always less than
a hundred microseconds or so? If the answer is "yes", then you
should also be willing to use rcu_read_lock(). If the answer is
"no", proceed to the next question.

3. Is this critical section executed infrequently (say, less than once
per few jiffies, on average), and is the critical section's execution time less than a jiffy or so? If the answer is "yes", then you should also be willing to use rcu_read_lock(). If the answer is "no", proceed to the next question.

4. Did you get here? If so, you probably don't want to use
rcu_read_lock(). Consider using something like srcu_read_lock(), reference counts, or sleeplocks instead.

Of course, these questions should be regarded as rough rules of thumb. And for every rough rule of thumb, there will be exceptions. These questions should still serve as a good general guide.

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 334

Quoted 7 lines

virt/kvm/eventfd.c +5 -0

+	rcu_read_lock();
+
+	list_for_each_entry_rcu(item, &group->items, list) {
+		if (iosignalfd_is_match(group, item, val, len))
+			eventfd_signal(item->file, 1);

From what I can see, this code could invoke eventfd_signal() on an item that just got list_del_rcu()ed. Is this really OK?

(No idea whether or not it is OK myself. Looks like we might change some state in the underlying file and attempt to wake up some processes. Might be OK, might not, depending on what you are up to.)

Quoted 153 lines

virt/kvm/eventfd.c +152 -0

+	}
+
+	rcu_read_unlock();
+}
+
+/*
+ * MMIO/PIO reads against the group indiscriminately return all zeros
+ */
+static void
+iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
+		      void *val)
+{
+	memset(val, 0, len);
+}
+
+static void
+_iosignalfd_group_destructor(struct _iosignalfd_group *group)
+{
+	list_del(&group->list);
+	kfree(group);
+}
+
+static void
+iosignalfd_group_destructor(struct kvm_io_device *this)
+{
+	struct _iosignalfd_group *group = to_group(this);
+
+	_iosignalfd_group_destructor(group);
+}
+
+/* assumes kvm->lock held */
+static struct _iosignalfd_group *
+iosignalfd_group_find(struct kvm *kvm, u64 addr)
+{
+	struct _iosignalfd_group *group;
+
+	list_for_each_entry(group, &kvm->iosignalfds, list) {
+		if (group->addr == addr)
+			return group;
+	}
+
+	return NULL;
+}
+
+static const struct kvm_io_device_ops iosignalfd_ops = {
+	.read       = iosignalfd_group_read,
+	.write      = iosignalfd_group_write,
+	.in_range   = iosignalfd_group_in_range,
+	.destructor = iosignalfd_group_destructor,
+};
+
+/*
+ * Atomically find an existing group, or create a new one if it doesn't already
+ * exist.
+ *
+ * assumes kvm->lock is held
+ */
+static struct _iosignalfd_group *
+iosignalfd_group_get(struct kvm *kvm, struct kvm_io_bus *bus,
+		      u64 addr, size_t len)
+{
+	struct _iosignalfd_group *group;
+
+	group = iosignalfd_group_find(kvm, addr);
+	if (!group) {
+		int ret;
+
+		group = kzalloc(sizeof(*group), GFP_KERNEL);
+		if (!group)
+			return ERR_PTR(-ENOMEM);
+
+		INIT_LIST_HEAD(&group->list);
+		INIT_LIST_HEAD(&group->items);
+		group->addr   = addr;
+		group->length = len;
+		kvm_iodevice_init(&group->dev, &iosignalfd_ops);
+
+		ret = kvm_io_bus_register_dev(bus, &group->dev);
+		if (ret < 0) {
+			kfree(group);
+			return ERR_PTR(ret);
+		}
+
+		list_add_tail(&group->list, &kvm->iosignalfds);
+
+	} else if (group->length != len)
+		/*
+		 * Existing groups must have the same addr/len tuple or we
+		 * reject the request
+		 */
+		return ERR_PTR(-EINVAL);
+
+	return group;
+}
+
+static int
+kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group = NULL;
+	struct _iosignalfd_item  *item = NULL;
+	struct file              *file;
+	int                       ret;
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		return ret;
+	}
+
+	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	if (!item) {
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	INIT_LIST_HEAD(&item->list);
+	item->file = file;
+
+	/*
+	 * Registering a "trigger" address is optional.  If this flag
+	 * is not specified, we leave the item->match pointer NULL, which
+	 * indicates a wildcard
+	 */
+	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER) {
+		if (args->len > sizeof(u64)) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		item->match = kzalloc(args->len, GFP_KERNEL);
+		if (!item->match) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		if (copy_from_user(item->match,
+				   (void *)args->trigger,
+				   args->len)) {
+			ret = -EFAULT;
+			goto fail;
+		}
+	}
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_get(kvm, bus, args->addr, args->len);
+	if (IS_ERR(group)) {
+		ret = PTR_ERR(group);
+		mutex_unlock(&kvm->lock);
+		goto fail;

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 496

Quoted 10 lines

virt/kvm/eventfd.c +10 -0

+	}
+
+	/*
+	 * Note: We are committed to succeed at this point since we have
+	 * (potentially) published a new group-device.  Any failure handling
+	 * added in the future after this point will need to be handled
+	 * carefully.
+	 */
+
+	list_add_tail_rcu(&item->list, &group->items);

Good, protected by kvm->lock.

Quoted 49 lines

virt/kvm/eventfd.c +49 -0

+	mutex_unlock(&kvm->lock);
+
+	return 0;
+
+fail:
+	if (item) {
+		/*
+		 * it would have never made it to the group->items list
+		 * in the failure path, so we dont need to worry about removing
+		 * it
+		 */
+		kfree(item->match);
+		kfree(item);
+	}
+
+	if (file)
+		fput(file);
+
+	return ret;
+}
+
+static void
+iosignalfd_item_free(struct rcu_head *rhp)
+{
+	struct _iosignalfd_item *item = to_item(rhp);
+
+	fput(item->file);
+	kfree(item->match);
+	kfree(item);
+}
+
+static int
+kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group;
+	struct _iosignalfd_item  *item, *tmp;
+	struct file              *file;
+	int                       ret = 0;
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_find(kvm, args->addr);
+	if (!group) {
+		ret = -EINVAL;
+		goto out;
+	}
+

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 553

Quoted 8 lines

virt/kvm/eventfd.c +7 -0

+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out;
+	}
+
+	list_for_each_entry_safe(item, tmp, &group->items, list) {

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 561

Quoted 9 lines

virt/kvm/eventfd.c +8 -0

+		/*
+		 * any items registered at this group-address with the matching
+		 * eventfd will be removed
+		 */
+		if (item->file != file)
+			continue;
+
+		list_del_rcu(&item->list);

Good, also protected by kvm->lock.

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 562

Quoted 2 lines

virt/kvm/eventfd.c +1 -0

+		call_rcu(&item->rcu, iosignalfd_item_free);

And also good here, deferring free of the RCU-protected structures.

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 571

Quoted 11 lines

virt/kvm/eventfd.c +9 -0

+	}
+
+	if (list_empty(&group->items)) {
+		/*
+		 * We should unpublish our group device if we just removed
+		 * the last of its contained items
+		 */
+		kvm_io_bus_unregister_dev(bus, &group->dev);
+		_iosignalfd_group_destructor(group);

Quoted 41 lines

virt/kvm/eventfd.c +18 -0

+	}
+
+	fput(file);
+
+out:
+	mutex_unlock(&kvm->lock);
+
+	return ret;
+}
+
+int
+kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
+		return kvm_deassign_iosignalfd(kvm, args);
+
+	return kvm_assign_iosignalfd(kvm, args);
+}

virt/kvm/kvm_main.c +7 -1

index 179c650..91d0fe2 100644
@@ -977,7 +977,7 @@ static struct kvm *kvm_create_vm(void)
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
-	kvm_irqfd_init(kvm);
+	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
 	init_rwsem(&kvm->slots_lock);

@@ -2215,6 +2215,15 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
 		break;
 	}
+	case KVM_IOSIGNALFD: {
+		struct kvm_iosignalfd entry;
+
+		r = -EFAULT;
+		if (copy_from_user(&entry, argp, sizeof entry))
+			goto out;

virt/kvm/kvm_main.c static long kvm_vm_ioctl(struct file *filp, · line 2229

Quoted 8 lines

virt/kvm/kvm_main.c +3 -0

+		r = kvm_iosignalfd(kvm, &entry);
+		break;
+	}
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
 	}

Quoted 255 lines · Paul E. McKenney

Quoted 57 lines · Gregory Haskins

iosignalfd is a mechanism to register PIO/MMIO regions to trigger an eventfd signal when written to by a guest. Host userspace can register any arbitrary IO address with a corresponding eventfd and then pass the eventfd to a specific end-point of interest for handling.

Normal IO requires a blocking round-trip since the operation may cause side-effects in the emulated model or may return data to the caller. Therefore, an IO in KVM traps from the guest to the host, causes a VMX/SVM "heavy-weight" exit back to userspace, and is ultimately serviced by qemu's device model synchronously before returning control back to the vcpu.

However, there is a subclass of IO which acts purely as a trigger for other IO (such as to kick off an out-of-band DMA request, etc). For these patterns, the synchronous call is particularly expensive since we really only want to simply get our notification transmitted asychronously and return as quickly as possible. All the sychronous infrastructure to ensure proper data-dependencies are met in the normal IO case are just unecessary overhead for signalling. This adds additional computational load on the system, as well as latency to the signalling path.

Therefore, we provide a mechanism for registration of an in-kernel trigger point that allows the VCPU to only require a very brief, lightweight exit just long enough to signal an eventfd. This also means that any clients compatible with the eventfd interface (which includes userspace and kernelspace equally well) can now register to be notified. The end result should be a more flexible and higher performance notification API for the backend KVM hypervisor and perhipheral components.

To test this theory, we built a test-harness called "doorbell". This module has a function called "doorbell_ring()" which simply increments a counter for each time the doorbell is signaled. It supports signalling from either an eventfd, or an ioctl().

We then wired up two paths to the doorbell: One via QEMU via a registered io region and through the doorbell ioctl(). The other is direct via iosignalfd.

You can download this test harness here:

ftp://ftp.novell.com/dev/ghaskins/doorbell.tar.bz2

The measured results are as follows:

qemu-mmio:       110000 iops, 9.09us rtt
iosignalfd-mmio: 200100 iops, 5.00us rtt
iosignalfd-pio:  367300 iops, 2.72us rtt

I didn't measure qemu-pio, because I have to figure out how to register a PIO region with qemu's device model, and I got lazy. However, for now we can extrapolate based on the data from the NULLIO runs of +2.56us for MMIO, and -350ns for HC, we get:

qemu-pio:      153139 iops, 6.53us rtt
iosignalfd-hc: 412585 iops, 2.37us rtt

these are just for fun, for now, until I can gather more data.

Quoted 9 lines

Here is a graph for your convenience:

http://developer.novell.com/wiki/images/7/76/Iofd-chart.png

The conclusion to draw is that we save about 4us by skipping the userspace hop.

--------------------

One set of decision criteria for RCU vs. SRCU below, and one question about the RCU reader running concurrently with a deletion. Looks pretty good, in general!

Thanx, Paul

Quoted 179 lines

Signed-off-by Gregory Haskins <[email protected]>

---

 arch/x86/kvm/x86.c       |    1 
 include/linux/kvm.h      |   15 ++
 include/linux/kvm_host.h |   10 +
 virt/kvm/eventfd.c       |  356 ++++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      |   11 +
 5 files changed, 389 insertions(+), 4 deletions(-)

arch/x86/kvm/x86.c +1 -0

index c1ed485..c96c0e3 100644
@@ -1097,6 +1097,7 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IRQ_INJECT_STATUS:
 	case KVM_CAP_ASSIGN_DEV_IRQ:
 	case KVM_CAP_IRQFD:
+	case KVM_CAP_IOSIGNALFD:
 	case KVM_CAP_PIT2:
 		r = 1;
 		break;

include/linux/kvm.h +15 -0

index 632a856..53b720d 100644
@@ -300,6 +300,19 @@ struct kvm_guest_debug {
 	struct kvm_guest_debug_arch arch;
 };
 
+#define KVM_IOSIGNALFD_FLAG_TRIGGER   (1 << 0)
+#define KVM_IOSIGNALFD_FLAG_PIO       (1 << 1)
+#define KVM_IOSIGNALFD_FLAG_DEASSIGN  (1 << 2)
+
+struct kvm_iosignalfd {
+	__u64 trigger;
+	__u64 addr;
+	__u32 len;
+	__u32 fd;
+	__u32 flags;
+	__u8  pad[36];
+};
+
 #define KVM_TRC_SHIFT           16
 /*
  * kvm trace categories

@@ -430,6 +443,7 @@ struct kvm_trace_rec {
 #ifdef __KVM_HAVE_PIT
 #define KVM_CAP_PIT2 33
 #endif
+#define KVM_CAP_IOSIGNALFD 34
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -537,6 +551,7 @@ struct kvm_irqfd {
 #define KVM_DEASSIGN_DEV_IRQ       _IOW(KVMIO, 0x75, struct kvm_assigned_irq)
 #define KVM_IRQFD                  _IOW(KVMIO, 0x76, struct kvm_irqfd)
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
+#define KVM_IOSIGNALFD             _IOW(KVMIO, 0x78, struct kvm_iosignalfd)
 
 /*
  * ioctls for vcpu fds

include/linux/kvm_host.h +8 -2

index 216fe07..b705960 100644
@@ -138,6 +138,7 @@ struct kvm {
 	struct kvm_io_bus pio_bus;
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 	struct list_head irqfds;
+	struct list_head iosignalfds;
 #endif
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;

@@ -533,19 +534,24 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
 
 #ifdef CONFIG_HAVE_KVM_EVENTFD
 
-void kvm_irqfd_init(struct kvm *kvm);
+void kvm_eventfd_init(struct kvm *kvm);
 int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
 void kvm_irqfd_release(struct kvm *kvm);
+int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args);
 
 #else
 
-static inline void kvm_irqfd_init(struct kvm *kvm) {}
+static inline void kvm_eventfd_init(struct kvm *kvm) {}
 static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
 {
 	return -EINVAL;
 }
 
 static inline void kvm_irqfd_release(struct kvm *kvm) {}
+static inline int kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	return -EINVAL;
+}
 
 #endif /* CONFIG_HAVE_KVM_EVENTFD */
 

virt/kvm/eventfd.c +46 -1

index f3f2ea1..77befb3 100644
@@ -21,6 +21,7 @@
  */
 
 #include <linux/kvm_host.h>
+#include <linux/kvm.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
 #include <linux/wait.h>

@@ -29,6 +30,8 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 
+#include "iodev.h"
+
 /*
  * --------------------------------------------------------------------
  * irqfd: Allows an fd to be used to inject an interrupt to the guest

@@ -208,9 +211,10 @@ kvm_deassign_irqfd(struct kvm *kvm, int fd, int gsi)
 }
 
 void
-kvm_irqfd_init(struct kvm *kvm)
+kvm_eventfd_init(struct kvm *kvm)
 {
 	INIT_LIST_HEAD(&kvm->irqfds);
+	INIT_LIST_HEAD(&kvm->iosignalfds);
 }
 
 int

@@ -233,3 +237,353 @@ kvm_irqfd_release(struct kvm *kvm)
 		irqfd_release(irqfd);
 	}
 }
+
+/*
+ * --------------------------------------------------------------------
+ * iosignalfd: translate a PIO/MMIO memory write to an eventfd signal.
+ *
+ * userspace can register a PIO/MMIO address with an eventfd for recieving
+ * notification when the memory has been touched.
+ * --------------------------------------------------------------------
+ */
+
+/*
+ * Design note: We create one PIO/MMIO device (iosignalfd_group) which
+ * aggregates  one or more iosignalfd_items.  Each item points to exactly one
+ * eventfd, and can be registered to trigger on any write to the group
+ * (wildcard), or to a write of a specific value.  If more than one item is to
+ * be supported, the addr/len ranges must all be identical in the group.  If a
+ * trigger value is to be supported on a particular item, the group range must
+ * be exactly the width of the trigger.
+ */
+
+struct _iosignalfd_item {
+	struct list_head     list;
+	struct file         *file;
+	unsigned char       *match;
+	struct rcu_head      rcu;
+};
+
+struct _iosignalfd_group {
+	struct list_head     list;
+	u64                  addr;
+	size_t               length;
+	struct list_head     items;
+	struct kvm_io_device dev;
+};
+
+static inline struct _iosignalfd_group *to_group(struct kvm_io_device *dev)
+{
+	return container_of(dev, struct _iosignalfd_group, dev);
+}
+
+static inline struct _iosignalfd_item *to_item(struct rcu_head *rhp)

Quoted 9 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 283

Quoted 4 lines

virt/kvm/eventfd.c +3 -0

+{
+	return container_of(rhp, struct _iosignalfd_item, rcu);
+}

Given that you only use to_item() in one place, not clear to me that it is a win compared to open-coding it. In contrast, to_group() is used in several places, so it makes sense to define it.

Yeah, I think open-coding it resulted in a 80+ line that I had to break, and I hate that. So I went for consistency and tidiness instead, if but a tad overkill. ;)

Quoted 66 lines

Quoted 37 lines

virt/kvm/eventfd.c +35 -0

+static int
+iosignalfd_group_in_range(struct kvm_io_device *this, gpa_t addr, int len,
+			  int is_write)
+{
+	struct _iosignalfd_group *p = to_group(this);
+
+	return ((addr >= p->addr && (addr < p->addr + p->length)));
+}
+
+static int
+iosignalfd_is_match(struct _iosignalfd_group *group,
+		    struct _iosignalfd_item *item,
+		    const void *val,
+		    int len)
+{
+	if (!item->match)
+		/* wildcard is a hit */
+		return true;
+
+	if (len != group->length)
+		/* mis-matched length is a miss */
+		return false;
+
+	/* otherwise, we have to actually compare the data */
+	return !memcmp(item->match, val, len) ? true : false;
+}
+
+/*
+ * MMIO/PIO writes trigger an event (if the data matches).
+ *
+ * This is invoked by the io_bus subsystem in response to an address match
+ * against the group.  We must then walk the list of individual items to check
+ * for a match and, if applicable, to send the appropriate signal. If the item
+ * in question does not have a "match" pointer, it is considered a wildcard
+ * and will always generate a signal.  There can be an arbitrary number

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 329

Quoted 10 lines

virt/kvm/eventfd.c +10 -0

+ * of distinct matches or wildcards per group.
+ */
+static void
+iosignalfd_group_write(struct kvm_io_device *this, gpa_t addr, int len,
+		       const void *val)
+{
+	struct _iosignalfd_group *group = to_group(this);
+	struct _iosignalfd_item *item;
+
+	/* FIXME: We should probably use SRCU */

Some test questions to help with this decision:

1. Ignoring lock-contention issues, would you be willing to hold a
spinlock_t across this critical section? If the answer is "yes", then you should also be willing to use rcu_read_lock(). If the answer is "no", proceed to the next question.

2. Is this critical section's execution time almost always less than
a hundred microseconds or so? If the answer is "yes", then you
should also be willing to use rcu_read_lock(). If the answer is
"no", proceed to the next question.

3. Is this critical section executed infrequently (say, less than once
per few jiffies, on average), and is the critical section's execution time less than a jiffy or so? If the answer is "yes", then you should also be willing to use rcu_read_lock(). If the answer is "no", proceed to the next question.

Quoted 8 lines

4. Did you get here? If so, you probably don't want to use
rcu_read_lock(). Consider using something like srcu_read_lock(), reference counts, or sleeplocks instead.

Of course, these questions should be regarded as rough rules of thumb. And for every rough rule of thumb, there will be exceptions. These questions should still serve as a good general guide.

Actually as it stands right now the eventfd_signal() takes a spin_lock_irq() for its duration, so a basic rcu_read_lock() may be in the noise (at least for now). I'd like to work with Davide at some point in the future to change eventfd internals to allow signalling callbacks to be optionally preemptible (at least when they are called from preemptible code to begin with), but we can address this code path at that time.

The biggest caveat here is that while today the eventfd_signal() unavoidably introduces its own non-preemptible section, iosignalfd could conceivably be composed of an arbitrarily long list of iosignalfd_items. We would therefore be extending this CS out a bit further than the single eventfd_signal overhead alone would imply. That might be justification in of itself to use something more friendly, like srcu.

Out of curiosity, how much more expensive is an srcu read-side CS compared to plain rcu? How about compared to an atomic (like atomic_inc)? Don't go off running some microbenchmarks just for me. I am just wondering if you have measured it in the past and know the relative numbers off the top of your head.

Quoted 10 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 334

Quoted 6 lines

virt/kvm/eventfd.c +5 -0

+	rcu_read_lock();
+
+	list_for_each_entry_rcu(item, &group->items, list) {
+		if (iosignalfd_is_match(group, item, val, len))
+			eventfd_signal(item->file, 1);

From what I can see, this code could invoke eventfd_signal() on an item that just got list_del_rcu()ed. Is this really OK?

If I understand the question, this is similar to the situation with the irqfd review we just did: that is, there is a window of time between unlinking the object and its destruction, and the question is whether its still ok to use the object during this window?

If that is true, the answer is "yes". As in the irqfd case, I really only care about making sure that we cannot invoke the free()/fput() function while this critical section referenced above is running. We hold a valid reference to item->file until the point that the iosignal_item_free() method is called (via call_rcu), so the eventfd_signal() should be ok as long as it completes before then.

Quoted 225 lines

(No idea whether or not it is OK myself. Looks like we might change some state in the underlying file and attempt to wake up some processes. Might be OK, might not, depending on what you are up to.)

Quoted 153 lines

virt/kvm/eventfd.c +152 -0

+	}
+
+	rcu_read_unlock();
+}
+
+/*
+ * MMIO/PIO reads against the group indiscriminately return all zeros
+ */
+static void
+iosignalfd_group_read(struct kvm_io_device *this, gpa_t addr, int len,
+		      void *val)
+{
+	memset(val, 0, len);
+}
+
+static void
+_iosignalfd_group_destructor(struct _iosignalfd_group *group)
+{
+	list_del(&group->list);
+	kfree(group);
+}
+
+static void
+iosignalfd_group_destructor(struct kvm_io_device *this)
+{
+	struct _iosignalfd_group *group = to_group(this);
+
+	_iosignalfd_group_destructor(group);
+}
+
+/* assumes kvm->lock held */
+static struct _iosignalfd_group *
+iosignalfd_group_find(struct kvm *kvm, u64 addr)
+{
+	struct _iosignalfd_group *group;
+
+	list_for_each_entry(group, &kvm->iosignalfds, list) {
+		if (group->addr == addr)
+			return group;
+	}
+
+	return NULL;
+}
+
+static const struct kvm_io_device_ops iosignalfd_ops = {
+	.read       = iosignalfd_group_read,
+	.write      = iosignalfd_group_write,
+	.in_range   = iosignalfd_group_in_range,
+	.destructor = iosignalfd_group_destructor,
+};
+
+/*
+ * Atomically find an existing group, or create a new one if it doesn't already
+ * exist.
+ *
+ * assumes kvm->lock is held
+ */
+static struct _iosignalfd_group *
+iosignalfd_group_get(struct kvm *kvm, struct kvm_io_bus *bus,
+		      u64 addr, size_t len)
+{
+	struct _iosignalfd_group *group;
+
+	group = iosignalfd_group_find(kvm, addr);
+	if (!group) {
+		int ret;
+
+		group = kzalloc(sizeof(*group), GFP_KERNEL);
+		if (!group)
+			return ERR_PTR(-ENOMEM);
+
+		INIT_LIST_HEAD(&group->list);
+		INIT_LIST_HEAD(&group->items);
+		group->addr   = addr;
+		group->length = len;
+		kvm_iodevice_init(&group->dev, &iosignalfd_ops);
+
+		ret = kvm_io_bus_register_dev(bus, &group->dev);
+		if (ret < 0) {
+			kfree(group);
+			return ERR_PTR(ret);
+		}
+
+		list_add_tail(&group->list, &kvm->iosignalfds);
+
+	} else if (group->length != len)
+		/*
+		 * Existing groups must have the same addr/len tuple or we
+		 * reject the request
+		 */
+		return ERR_PTR(-EINVAL);
+
+	return group;
+}
+
+static int
+kvm_assign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group = NULL;
+	struct _iosignalfd_item  *item = NULL;
+	struct file              *file;
+	int                       ret;
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		return ret;
+	}
+
+	item = kzalloc(sizeof(*item), GFP_KERNEL);
+	if (!item) {
+		ret = -ENOMEM;
+		goto fail;
+	}
+
+	INIT_LIST_HEAD(&item->list);
+	item->file = file;
+
+	/*
+	 * Registering a "trigger" address is optional.  If this flag
+	 * is not specified, we leave the item->match pointer NULL, which
+	 * indicates a wildcard
+	 */
+	if (args->flags & KVM_IOSIGNALFD_FLAG_TRIGGER) {
+		if (args->len > sizeof(u64)) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		item->match = kzalloc(args->len, GFP_KERNEL);
+		if (!item->match) {
+			ret = -ENOMEM;
+			goto fail;
+		}
+
+		if (copy_from_user(item->match,
+				   (void *)args->trigger,
+				   args->len)) {
+			ret = -EFAULT;
+			goto fail;
+		}
+	}
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_get(kvm, bus, args->addr, args->len);
+	if (IS_ERR(group)) {
+		ret = PTR_ERR(group);
+		mutex_unlock(&kvm->lock);
+		goto fail;

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 496

Quoted 10 lines

virt/kvm/eventfd.c +10 -0

+	}
+
+	/*
+	 * Note: We are committed to succeed at this point since we have
+	 * (potentially) published a new group-device.  Any failure handling
+	 * added in the future after this point will need to be handled
+	 * carefully.
+	 */
+
+	list_add_tail_rcu(&item->list, &group->items);

Good, protected by kvm->lock.

Quoted 53 lines

virt/kvm/eventfd.c +53 -0

+	mutex_unlock(&kvm->lock);
+
+	return 0;
+
+fail:
+	if (item) {
+		/*
+		 * it would have never made it to the group->items list
+		 * in the failure path, so we dont need to worry about removing
+		 * it
+		 */
+		kfree(item->match);
+		kfree(item);
+	}
+
+	if (file)
+		fput(file);
+
+	return ret;
+}
+
+static void
+iosignalfd_item_free(struct rcu_head *rhp)
+{
+	struct _iosignalfd_item *item = to_item(rhp);
+
+	fput(item->file);
+	kfree(item->match);
+	kfree(item);
+}
+
+static int
+kvm_deassign_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	int                       pio = args->flags & KVM_IOSIGNALFD_FLAG_PIO;
+	struct kvm_io_bus        *bus = pio ? &kvm->pio_bus : &kvm->mmio_bus;
+	struct _iosignalfd_group *group;
+	struct _iosignalfd_item  *item, *tmp;
+	struct file              *file;
+	int                       ret = 0;
+
+	mutex_lock(&kvm->lock);
+
+	group = iosignalfd_group_find(kvm, args->addr);
+	if (!group) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	file = eventfd_fget(args->fd);
+	if (IS_ERR(file)) {
+		ret = PTR_ERR(file);
+		goto out;

Quoted 4 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 553

Quoted 4 lines

virt/kvm/eventfd.c +3 -0

+	}
+
+	list_for_each_entry_safe(item, tmp, &group->items, list) {

Just to be sure, its ok to use a non-rcu list iterator in conjunction with the list_add/del_rcu mutating primitives as long as I provide my own mutal exclusion (e.g. kvm->lock), right?

Quoted 13 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 561

Quoted 9 lines

virt/kvm/eventfd.c +8 -0

+		/*
+		 * any items registered at this group-address with the matching
+		 * eventfd will be removed
+		 */
+		if (item->file != file)
+			continue;
+
+		list_del_rcu(&item->list);

Good, also protected by kvm->lock.

Quoted 5 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 562

Quoted 2 lines

virt/kvm/eventfd.c +1 -0

+		call_rcu(&item->rcu, iosignalfd_item_free);

And also good here, deferring free of the RCU-protected structures.

Out of curiosity: Would call_rcu() work if we were using srcu instead? Or is my only choice a blocking synchronize_srcu() here? I couldn't seem to find a call_srcu().

Quoted 7 lines

Quoted 7 lines

virt/kvm/eventfd.c +7 -0

+	}
+
+	if (list_empty(&group->items)) {
+		/*
+		 * We should unpublish our group device if we just removed
+		 * the last of its contained items
+		 */

Quoted 3 lines

virt/kvm/eventfd.c kvm_irqfd_release(struct kvm *kvm) · line 571

Quoted 3 lines

virt/kvm/eventfd.c +2 -0

+		kvm_io_bus_unregister_dev(bus, &group->dev);
+		_iosignalfd_group_destructor(group);

I think I screwed this part up. I should technically defer the destruction of the group object until after the next grace as well, right? (We are using its groups->items list in the read-side CS). Otherwise its a race, I think.

Quoted 41 lines

Quoted 41 lines

virt/kvm/eventfd.c +18 -0

+	}
+
+	fput(file);
+
+out:
+	mutex_unlock(&kvm->lock);
+
+	return ret;
+}
+
+int
+kvm_iosignalfd(struct kvm *kvm, struct kvm_iosignalfd *args)
+{
+	if (args->flags & KVM_IOSIGNALFD_FLAG_DEASSIGN)
+		return kvm_deassign_iosignalfd(kvm, args);
+
+	return kvm_assign_iosignalfd(kvm, args);
+}

virt/kvm/kvm_main.c +7 -1

index 179c650..91d0fe2 100644
@@ -977,7 +977,7 @@ static struct kvm *kvm_create_vm(void)
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
-	kvm_irqfd_init(kvm);
+	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
 	init_rwsem(&kvm->slots_lock);

@@ -2215,6 +2215,15 @@ static long kvm_vm_ioctl(struct file *filp,
 		r = kvm_irqfd(kvm, data.fd, data.gsi, data.flags);
 		break;
 	}
+	case KVM_IOSIGNALFD: {
+		struct kvm_iosignalfd entry;
+
+		r = -EFAULT;
+		if (copy_from_user(&entry, argp, sizeof entry))
+			goto out;

Quoted 8 lines

virt/kvm/kvm_main.c static long kvm_vm_ioctl(struct file *filp, · line 2229

Quoted 8 lines

virt/kvm/kvm_main.c +3 -0

+		r = kvm_iosignalfd(kvm, &entry);
+		break;
+	}
 	default:
 		r = kvm_arch_vm_ioctl(filp, ioctl, arg);
 	}

Thanks again, Paul!
-Greg

(Applies to kvm.git/master:25deed73)

This is v5 of the series. For more details, please see the header to patch 2/2.

This series has been tested against the kvm-eventfd unit test, and appears to be functioning properly. You can download this test here:

ftp://ftp.novell.com/dev/ghaskins/kvm-eventfd.tar.bz2

This series is ready to be considered for inclusion, pending any further review comments.

[
Changelog:

v5:
*) Removed "cookie" field, which was a misunderstanding on my
part on what Avi wanted for a data-match feature
*) Added a new "trigger" data-match feature which I think is
much closer to what we need.
*) We retain the dev_count field in the io_bus infrastructure
and instead back-fill the array on removal.
*) Various minor cleanups
*) Rebased to kvm.git/master:25deed73

v4:
*) Fixed a bug in the original 2/4 where the PIT failure case
would potentially leave the io_bus components registered.
*) Condensed the v3 2/4 and 3/4 into one patch (2/2) since
the patches became interdependent with the fix described above
*) Rebased to kvm.git/master:74dfca0a

v3:
*) fixed patch 2/4 to handle error cases instead of BUG_ON *) implemented same HAVE_EVENTFD protection mechanism as
irqfd to prevent compilation errors on unsupported arches
*) completed testing
*) rebased to kvm.git/master:7391a6d5

v2:
*) added optional data-matching capability (via cookie field) *) changed name from iofd to iosignalfd
*) added io_bus unregister function
*) implemented deassign feature

v1:
*) original release (integrated into irqfd v7 series as "iofd")
]

---

Gregory Haskins (2):
kvm: add iosignalfd support
kvm: make io_bus interface more robust

 arch/x86/kvm/i8254.c      |   22 +++
 arch/x86/kvm/i8259.c      |    9 +
 arch/x86/kvm/x86.c        |    1 
 include/linux/kvm.h       |   15 ++
 include/linux/kvm_host.h  |   16 ++
 virt/kvm/coalesced_mmio.c |    8 +
 virt/kvm/eventfd.c        |  356 +++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/ioapic.c         |    9 +
 virt/kvm/kvm_main.c       |   41 +++++
 9 files changed, 462 insertions(+), 15 deletions(-)

Marcello, Avi, and myself have previously agreed that Marcello's
mmio-locking cleanup should go in first. When that happens, I will
need to rebase this series because it changes how you interface to the io_bus code. I should have mentioned that here, but forgot. (Speaking of, is there an ETA when that code will be merged Avi?)